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DIGITAL QUARTZ PRESSURE TRANSDUCERS for FLIGHT APPLICATIONS Jerome M

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DIGITAL QUARTZ PRESSURE TRANSDUCERS for FLIGHT APPLICATIONS Jerome M DIGITAL QUARTZ PRESSURE TRANSDUCERS FOR FLIGHT APPLICATIONS Jerome M. Paros Paroscientific, Inc. INTRODUCTION A series of high precision pressure transducers has been developed to meet the requirements of a variety of aerospace applications. The development of these transducers was prompted by the widespread use and increasing trend toward digital data-acquisition and control systems. The design and performance goals included the requirements for a digital-type output, high accuracy, low power consumption, exceptional reliability, and small size and weight. Also, simple mathematical characterization in the processing of the output signals and general insensitivity to the environmental errors of acceleration, vibration, temperature, humidity and electromagnetic interference, were important considerations. The design, construction and performance of the Digiquartz Pressure Transducers are described in the attached article1 from Measurements and Data entitled "Digital Pressure Transducers". The object of this paper is to describe some of the past, present and future aerospace applications related to flight systems. Topics to be discussed include digital electronic engine control systems, in-flight engine monitoring, flight performance benefits from improved instrumentation and control, and digital air data computer applications. DIGITAL ELECTRONIC ENGINE CONTROL The first flight application of the digital quartz pressure transducers was their use on an F-111 aircraft in the Integrated Propulsion Control System (IPCS). The IPCS program was a research and development effort in which one set of hydro-mechanical engine and inlet controls on a supersonic airplane was replaced with a digital electronic control system. This program was sponsored by the Aero Propulsion Laboratory, Air Force Systems Command, Wright-Patterson AFB, Ohio. The contract was awarded to Boeing Aerospace Company in March 1973- Major participants included Boeing, Pratt & Whitney Division of United Technologies Corporation and Honeywell Inc. Altitude cell tests were performed at NASA- Lewis Research Center and flight tests performed at NASA-Edwards Flight Research Center. The general configuration of the IPCS aircraft is shown in Figure 1. By integrating the inlet and engine controls as shown in the system schematic of Figure 2, the aircraft can operate closer to its performance limits while avoiding possible adverse interactions between engine, inlet, and air frame. Advanced sensors and a digital computer/control system provide more accurate and stable control enabling the engine to develop greater thrust and optimized performance, resulting in extended engine life, greater fuel economy and reduced maintenance costs. Another advantage of digital electronic control systems is their inherent flexibility. Software programming changes to the digital computer can match standard digital hardware controls with a variety of engines, inlets and airframes. Additional benefits are possible due to the ability of the digital propulsion control system to communicate directly with other digital aircraft systems such as the flight controls and air data computer. The digital computer links the inlet and engine controls with a group of advanced sensors, including digital quartz pressure transducers used to measure inlet and output pressures as shown in Figures 3 and 4. A distortion rake supplied by NASA measures the pressure profiles at the fan face. Four digital quartz pressure transducers with ranges of 0 to 30 psia are used to measure inlet pressure and inlet distortion. The digital computer uses the output signals to control the system to accommodate the distortion in the airflow and prevent engine stall through solenoid bleed valves. Two transducers are located at the local mach probe to measure static and total pressure. Two 0 to 30 psia transducers measure static and total pressures at the duct exit. These transducer measurements feed into the computer controlling the spike and the cone on the variable inlet of this supersonic airplane, The flight test phase of the IPCS program was successfully completed in March, of 1976. Some conclusions that can be drawn from the IPCS program are that more precise control of a propulsion system is desirable and possible, but depends upon the availability of precise measurements made with accurate, reliable, and compatible sensors. Transducers with high reliability and outstanding performance are paramount requirements for aircraft control applications; however, significant design and analysis benefits can result from proper engine instrumentation and in- flight performance monitoring. IN FLIGHT ENGINE PERFORMANCE MONITORING Precise analysis of engine performance under flight conditions has been made possible through the use of the digital quartz pressure transducers. Figure 5 shows the general location and function of the diagnostic flight test pressure sensors employed on the Air Launched Cruise Missile (ALCM). The ALCM is a highly accurate, extended range, air to ground weapon that can be launched by a penetrating bomber such as the B-52 and the B-1. After the missile is ejected from its carry position, the engine inlet pops up and the elevons are deployed, followed next by the vertical tail. Then the low bypass turbofan engine is ignited, the wings are unfolded and full engine thrust is rapidly achieved as a function of altitude at launch. Five transducers are mounted in a single package on the engine bypass duct. These 0-45 psia sensors are used to measure inner and outer fan duct total pressures as well as inner, center and outer turbine exhaust pressures. One 0- 300 psia transducer measures compressor delivery pressures as mounted on the engine. Two 0-30 psia transducers are used at the inlet duct to measure total inlet pressures. These measurements in conjunction with other diagnostic information can determine basic engine performance. The requirements imposed on the pressure transducers included a digital-type output, high accuracy, fast response time, small size and weight, and the ability to perform well under the severe environmental conditions of shock, vibration, and temperature associated with this missile. These sensors have also been used as part of a thrust measurement system on the McDonnell- Douglas YC-15 prototype MSTOL transport, in which the Model 245-A transducers measure core engine discharge and fan discharge pressures. FLIGHT PERFORMANCE BENEFITS FROM IMPROVED INSTRUMENTATION AND CONTROL One of the most important flight benefits achievable through improved instrumentation is reduced fuel consumption. Because of the rise in the price of jet engine fuel, both aircraft manufacturers and users have reviewed methods of improving fuel conservation. An article published by the Boeing Commercial Aircraft Company has examined possible areas of improvement including reduced aerodynamic drag, and improved instrumentation such as mach meter reading and engine pressure ratio (EPR) transmission and display. The study concluded that the greatest potential for fuel savings on these commercial aircraft were in order of importance, inaccurate mach meter correction, improved EPR gauges and flow meters and improved aerodynamic performance. Contributions to improved fuel economy must be achieved not only through improved instrumentation, controls and displays, but also through proper maintenance and calibration of these devices. As an example of the penalties associated with instrument errors the effects due to inaccurate readings on 747 and 727 airplane mach meters were examined. This is shown in Table 1 where an assumed error in mach meter of 0.01 mach low (i.e. reading 0.84 mach when the airplane was in fact flying 0.85 mach). If the mach meter was inherently inaccurate or had been subjected to environmental errors, or had been inadequately calibrated, then the airplane would actually be flying 4 knots indicated air speed and 6 knots true air speed faster at cruise altitude Flying at a higher than optimum speed as a result of the instrument error results in a fuel burn penalty on a 747 aircraft of 717 lbs per hour, or 226,655 US gallons per year. Table 1 shows the associated penalties in dollars for the added fuel consumption based on fuel costs of 20 cents per gallon, 40 cents per gallon, and 60 cents per gallon. The respective dollar amounts which could be saved per year by correct mach meter reading for the 747 airplane is over $45,000, $90,000 and $135,000 per year based on the assumed mach meter error and the variable costs of the jet fuel. Comparative figures for a 727 airplane indicate penalties would be approximately one third as great as that for the 747 airplane. The second major instrument error affecting fuel consumption is due to inadequate engine pressure ratio (EPR) gauges. Aircraft turbine engines are used to generate the propulsive energy by imparting momentum to a gas. The gas used by turbine engines is a mixture of products of combustion and air raised to a high energy level by the process of combustion. The basic turbine engine consists of a compressor, burner, turbine and nozzle. The combination of compressor, burner and turbine is referred to as the gas generator. This term is used to describe the function of accepting air at a low energy level and producing a new gas (air plus products of combustion) at a high energy level. The gas generator thus provides the high-energy gas to the nozzle and results in the propulsive force or thrust used to propel the aircraft. The cockpit instrument used to display
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